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re: hot water for storing heat
20 aug 1996
steve s wrote:
>i've been thinking about storing heat (for home heating) with hot water.
>the question is how. the 1500 gallon polyethelene stock tanks are cheap
>($250-$300) but the manufacturer says they're only good to 120f...
i wonder who makes them, and what shape they are? i see a few in the
jade mountain catalog, but they are more expensive.
>1. what are some good high temp (say, 180f) tank options?
a concrete septic tank? a site-built ferrocement tank, as described in
s b watt's book? a plywood box lined with a single piece of epdm rubber?
some plastic 55 gallon drums full of water?
>2. what would be the lowest temp that water is useful for heating,
>assuming forced air heat and an in duct heat exchanger. is 100f a good
>number to calculate from?
how much heat does your house need, and what would the water temperature be,
after a few days without sun?
the 2' x 2' magic aire shw 2347 duct heat exchanger transfers 45k btu/hour
between 125 f water and 68 f air at 1400 cfm, with a 0.1" h20 pressure drop.
a smaller water-air temperature difference would probably make proportionally
smaller heat flow, eg 100 f water and 68 f air might make 25k btu/hour.
nick
then again, you might store water in one of these...
6" diam windmill -----
^
. .
. . . |
. . . . 3m
inner cone ---->. . . |
white inside. . . .
. . ------ -->. .<-- 7 in
. . | slot | . .
. . ------ . .<-- outer cone, black outside
................................................
| 2.5m |
two 22 gauge gl sheet cones were made and assembled as shown in the picture.
a 6 inch diameter fan windmill was fixed at the top of the outer cone as shown.
the solar heat (2.5 x 3/2 = 3.75 kw solar insolation) made the outer cone
66 c and the inner cone was 33 c. four adjustable slots 4.5 in x 30 in were
made in the outer cone to obtain maximum flow rate of convection current to
turn the windmill. the temperature difference between inlets and outlet was
27 c. the following results were obtained:
free running, 2700 rpm 67 watts, 970 rpm 92 watts, 390 rpm
mirrors were added to increase the insolation to about 5 kw. this produced
dt = 45-50 c. now velocity increased to 13 m/sec (29 mph), 3 times the
previous figure. output power was 250-300 w. this indicates an efficiency
of 250/5000 = 5%.
if we scale up to a (30 square meter = 20 kw average insolation) 300 square
foot dome house we should be able to see 250 x 20/5 = 1000 w, our expected
power output. putting an insulating plastic film over the gl sheet should
increase the output.
nick
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